':' means Conservative amino acids. As we know it's the replacement of an amino acid residue with another one with similar properties, so why do the amino acids with the ':' below it have different properties (so different colours) (look at link above).

Are the absolutely crucial amino acids the amino acids that haven't be substituted with anything (so the are identical)?

What does this question mean:

1. Give an example (or highlight) of one position where the same amino acid is not found in all the sequences, but a small, hydrophobic amino acid is present in each sequence. Also give an example of a one position where a negatively-charged amino acid is found in all the sequences. (6 marks)

If you want me to understand better, could you answer the first part of the question? Here's the sequence alignment that is needed to answer the question (link below):